The present invention is directed to an improved surface-emitting laser which avoids the disadvantages created by the formation of a positive thermal lens.
Surface-emitting lasers with a vertical resonator (vertical cavity surface emitting lasers or VCSELs) are known for instance from IEEE Communications Magazine, May 97, pp. 164-170. These lasers contain a semiconductor body and share the structural feature that the resonator axis and thus the light emission as well are effected perpendicular to a primary face of the semiconductor body.
The pump current is typically introduced into the semiconductor body via contact faces. It is known that it is possible to dispose a contact face in the form of a ring contact on one primary face of the semiconductor body, with the laser radiation being decoupled through this primary face.
From the ring contact, the pump current is carried directly to the active layer of the semiconductor body. To achieve a pump density and to keep the threshold current low, the pump current is concentrated on a small volume of the active layer, hereinafter called the xe2x80x9cactive volumexe2x80x9d, with the aid of an aperture-like current-constriction layer. The generation and amplification of the laser radiation takes place in this active volume.
With this geometry, the path of the pump current through the semiconductor body (current path) and the generated radiation field in the semiconductor body (light path) are in major parts coincident. As a result, the radiation field and in particular its spatial field distribution are affected by the pump current, since as a function of the pump current, the electrical heat loss and thus the temperature of the semiconductor body are varied. As the pump current increases, a radially symmetrical temperature profile develops in the resonator. Because of the dependency of the index of diffraction of the semiconductor body on the temperature, thermal indexing arises, also known as a positive thermal lens.
A positive thermal lens leads to a constriction in the fundamental mode of the resonator, so that the overlap between the fundamental mode and the active volume, and thus the amplification of the fundamental mode, are reduced. Spatial hole burning reduces the amplification of the fundamental mode further, to the point of extinction of the fundamental mode. This also promotes the stimulation of oscillations of unwanted higher modes, making a fundamental mode operation (single mode operation or SM) impossible when the pump current is high.
This disadvantage is ameliorated if the light path and current path are separated. VCSELs with a partial separation of the light path and current path are known, for instance from U.S. Pat. No. 6,044,100. The so-called lateral injection described therein entails high production effort and expense, since for its development, expensive mesa etching of the semiconductor body using an etch stop layer is necessary.
Also in the VCSEL described in U.S. Pat. No. 6,044,100, the size of the active volume is fixedly specified and is determined essentially by the inside diameter of the current-constriction layer, so that the VCSEL is optimized for only a single operating point.
One object of the present invention is to create a VCSEL with improved current carrying, which at the same time can be produced economically.
The object of the present invention is therefore to create a VCSEL with improved current carrying, which at the same time can be produced economically. This object is attained by a VCSEL as defined by claim 1. Advantageous refinements are the subject of the dependent claims.
This and other objects are attained in accordance with one aspect of the present invention directed to a VCSEL having a semiconductor body, which has a resonator volume with a resonator axis, an active layer that includes an active volume that generates a radiation, and a first and a second primary face. A first contact face is formed on the first primary face and a second contact face is formed on the second primary face. The generated radiation is decoupled at least in part through the first primary face. The pump current in a first region is guided from the first primary face to the active layer, outside the resonator volume, predominantly parallel to the resonator axis. In a second region, the pump current is carried to the active volume predominantly perpendicular to the resonator axis, and an insulation layer (11) is embodied between the first region and the resonator volume.
Since the pump current is largely kept away from the resonator volume, the temperature increase caused by the electrical power loss, and especially the resultant temperature gradient in the resonator, are reduced. Thus in the resonator volume, the development of a positive thermal lens is avoided. This more-uniform temperature behavior, which is due to the improved current carrying, highly advantageously prevents a constriction of the fundamental mode and the unwanted stimulation of oscillations of higher modes as the pump current increases.
A further factor is that because of electrical losses, a temperature increase in the semiconductor body takes place outside the resonator volume. This conversely leads to a negative thermal lens in the resonator, which likewise counteracts a constriction of the fundamental mode. To great advantage, the invention thus makes VCSELs with high efficiency, high slope efficiency, and a high quantum yield possible that can be operated in the fundamental mode over a wide pump current range.
In an especially preferred embodiment of the invention, the carrying of the pump current outside the resonator is achieved by providing that an electrical insulation layer is embodied between the current-carrying first region outside the resonator and the resonator itself. Thus the existing semiconductor body is advantageously used for carrying current. Moreover, this insulation structure can advantageously be integrated into existing VCSEL structures without major effort or expense.
In a preferred refinement of the invention, the insulation layer is embodied by ion implantation. Targeted ion implantation in the region to be insulated creates a high number of lattice defects, so that in this region, the semiconductor degrades toward the electrical insulator. This often-used and therefore readily available method of embodying the insulation layer is advantageously utilized in the invention.
Alternatively, the insulation layer can be embodied by means of an etched trench. The etched trench can remain open or can be filled with an insulation or absorber material. Absorber materials, besides their insulating property, have high absorption in the spectral range of the laser emission wavelength. This absorption in the outer region of the resonator suppresses higher modes and thus reinforces the fundamental mode operation.
Forming insulation layers in the course of producing semiconductor bodies is known and has stood the test of time, and thus it advantageously requires no special effort or expense. Moreover, a semiconductor body formed in this way can more easily be provided with highly conductive ohmic contacts than a semiconductor body with mesa etching, for instance.
A preferred refinement of the invention comprises embodying the contact face on the decoupling face annularly. In operation, the generated laser radiation is decoupled through the annular opening. The VCSEL thus formed has an excellent beam quality, and the ring contact advantageously causes only slight shading of the decoupling face.
In a particularly preferred refinement of the invention, the contact face is embodied annularly, and the contact face is not closed. Alternatively, the contact face can be embodied as U-shaped. A V-shaped embodiment of the contact is especially preferred.
These contact shapes will hereinafter be called xe2x80x9copen contactsxe2x80x9d, in contrast to a closed ring contact. Open contacts cause the size of the active volume to depend on the pump current, so that it becomes larger as the pump current increases. Thus a self-regulating behavior of the VCSEL is achieved, with the great advantage that the VCSEL is optimized not for a single operating point but rather for an operating range.
Advantageous contact shapes for the self-regulation are ring contacts with an unsplit ring, U-shaped contacts, and V-shaped contacts; in terms of the self-regulating properties, V-shaped contacts are especially advantageous.
In this refinement of the invention, the active volume is advantageously optimally adapted particularly for low pump currents as well, so that a thus-formed VCSEL is distinguished by a very low threshold current and high slope efficiency.
Another preferred embodiment of the invention comprises adapting the lateral cross section of the insulation layer to the shape of the contact face on the decoupling face.
The term xe2x80x9clateral cross sectionxe2x80x9d means a cross section with a sectional plane orthogonal to the resonator axis. This adaptation advantageously assures that the pump current, along the shape of the contact in the second region, is delivered uniformly radially to the active volume and thus reinforces the self-regulation mechanism.